U.S. Democratic presidential nominee Hillary Clinton has confirmed that she got "dizzy" and lost her balance at a 9/11 memorial over the weekend but said she recovered quickly and she's feeling much better now.

Clinton left the outdoor ceremony in New York early amid a late-summer heat wave and could be seen buckling and needing help getting into her van, where she said the air conditioning helped to revive her.

"I felt dizzy, and I did lose my balance for a minute, but once I got in [the van], once I could sit down, once I could cool off, once I had some water, I immediately started feeling better," she said in an interview on CNN on September 12.

After a day of rest, "I'm feeling so much better and obviously I should have gotten some rest sooner," she said, admitting she had ignored her doctor's "very wise advice" to take a break for five days after she was diagnosed with pneumonia on September 9.

"I just want to get this over and done with and get back on the trail as soon as possible," Clinton said, predicting she would get back to campaigning "in the next couple of days."

Clinton's illness forced her to cancel a two-day fund-raising swing through California scheduled for September 12 and 13.

Clinton said she didn't immediately disclose her pneumonia diagnosis because she "just didn't think it was going to be that big a deal."

Clinton said she had similar dizzy spells in the past.

"I think really only twice that I can recall. You know, it is something that has occurred a few times over the course of my life. I'm aware of it and usually can avoid it," she said.

Clinton's husband, former U.S. President Bill Clinton, told PBS earlier that she got "severely dehydrated" and felt faint "on more than one occasion" over the years, usually because she kept on working despite feeling ill.

"She's worked like a demon, as you know, as secretary of state, as a senator, and in the years since," he said.

Clinton's health scare raised worries about her fitness for one of the world's most demanding jobs and revived concerns about a tendency toward secrecy that has dogged her run for the White House.

But Clinton and her campaign have gone to great lengths to assure that her illness poses nothing of lasting concern.

"There's no other undisclosed condition. The pneumonia is the extent of it," Clinton spokesman Brian Fallon told MSNBC.

Worries that Clinton could have health problems were stoked by a previous incident in 2012, when a stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion. Doctors said they found a blood clot on the brain. Clinton later received the all-clear but rumors about her health are sometimes cited by her opponent, Republican nominee Donald Trump.